1. Field of the Invention
The present invention relates to a laser power control apparatus and method for use in an optical disk apparatus for recording information onto an optical disk having a plurality of layers, an optical disk apparatus for recording information onto the optical disk and a recording method for recording information onto the optical disk.
2. Description of the Related Art
As the conventional method for adjusting a power of a laser beam of an optical disk apparatus, a known method includes the steps of adjusting a power of the laser beam according to the temperature and the position along the radial direction of the optical disk, and generating a data table of the power of the laser beam.
For example, the Japanese laid-open publication No. 1-112551 describes a method for determining a power of the laser beam using the data table. This conventional technique will be described with reference to FIG. 12.
FIG. 12 is a block diagram showing a configuration of a conventional optical disk apparatus for recording information onto an optical disk 1.
The optical disk 1 includes a single layer 1a. A recording film is provided for the layer 1a. 
The optical pickup 10 converges the laser beam onto the recording film of the layer 1a. 
A part of the laser beam emitted from the laser element 12 is transmitted through the reflection plate 15 and is detected by the emitting light detector 13.
Further, the laser beam emitted from the laser element 12 is reflected by the reflection plate 15, is transmitted through the deflection hologram 16, and is converged onto the recording film of the optical disk 1 by the convergence lens 11.
The laser beam (the reflected light) reflected by the recording film of the optical disk 1 is transmitted through the convergence lens 11, is deflected by the deflection hologram 16, is transmitted through the reflection plate 15 and is detected by the reflected light detector 14.
The laser beam detected by the emitting light detector 13 is analog-to-digital converted by the AD converter 31 included in the DSP (Digital Signal Processor) 30. The output from the AD converter 31 is supplied to the power control section 32 included in the DSP 30.
The power control section 32 generates a laser driving value such that a power of the laser beam detected by the emitting light detector 13 approaches a control target level, and controls the laser element 12 in accordance with the laser driving value.
Such a control is achieved, for example, by the DA converter 33 included in the DSP 30, which digital-to-analog converts the laser driving value generated by the power control section 32, and supplies the output from the DA converter 33 to the driving circuit 2. The control target level is determined by the power adjustment section 43 and is set in the power control section 32.
The driving circuit 2 drives the laser element 12 in accordance with the output from the DA converter 33.
The laser beam detected by the reflected light detector 14 (i.e. reflected light) is analog-to-digital converted by the AD converter 41 included in the DSP 30. The output from the AD converter 41 is supplied to the asymmetry detection section 42 included in the DSP 30.
The asymmetry detection section 42 detects asymmetry of the reflected light (Ipk+Ibm−2Iavg)/(Ipk−Ibm) from the peak level Ipk, the bottom level Ibm and the average level Iavg, and outputs a signal indicating the asymmetry of the reflected light to the power adjustment section 43.
The power adjustment section 43 searches an optimal value of the power of the laser beam such that the asymmetry of the reflected light becomes a desired value (e.g. “0”).
With reference to FIG. 13, it will be described below the steps of the method for adjusting the power of the laser beam.
The power adjustment is made for the recording film of the optical disk 1 (S1).
Such a power adjustment is made, for example, by the power, control section 32 controlling the laser element 12 such that the laser beam having the power according to a predetermined control target level is irradiated towards the optical disk 1 and by the power adjustment section 43 adjusting the control target level such that the asymmetry of the reflected light from the optical disk 1 becomes a desired value (e.g. “0”). The control target level which has been adjusted by the power adjustment section 43 is set in the power control section 32.
The power adjustment section 43 updates a table value for the current temperature with the control target level which has been adjusted by the power adjustment section 43 (S2).
It is determined whether the temperature has been changed (S3). The output of the temperature detector 17 is analog-to-digital converted by the AD converter 45 included in the DSP 30. The output of the AD converter 45 is supplied to the power adjustment section 43. The power adjustment section 43 determines whether the temperature has been changed by monitoring the output of the AD converter 45, for example.
If it is determined that the temperature has been changed, then it is determined whether the control target level has been adjusted for the current temperature (S4). The power adjustment section 43 determines whether the control target level has been adjusted by retrieving the contents of the RAM 44, for example.
If it is determined that the control target level has been adjusted, then the power adjustment section 43 reads the control target level corresponding to the current temperature from the table stored in the RAM 44 (S5), and sets the control target level in the power control section 32 (S6).
If it is determined that the control target level has not been adjusted, then the step to be processed is returned back to step S1.
According to the method for adjusting the power of the laser beam mentioned above, it is possible to adjust the power of the laser beam for recording so that a reproduced signal is obtained such that the asymmetry of the reflected light becomes a desired value (e.g. “0”), when the power of the laser beam emitted from the laser element is changed due to the changes in temperature which are caused by the temperature of the environment where the optical disk apparatus is used or the heat generated during the operation of the optical disk apparatus.
However, according to the conventional recording method, in the case where the optical disk has a plurality of layers (e.g. two layers) with information recording films, it is impossible to record information onto the recording film of any layer other than the reference layer of the optical disk at the optimal laser power, when the temperature of the environment where the optical disk apparatus is used has been changed.
Specifically, even if a data table for one layer (i.e. the reference layer) is generated and stored in response to the changes in temperature, it is impossible to generate a data table for any layer other than the reference layer.
In order to generate the data table for a layer other than the reference layer, it is necessary to make a power adjustment to the layer other than the reference layer, after making a power adjustment to the reference layer. It requires time for moving between two layers and time for making the power adjustment to the layer other than the reference layer by recording/reproducing data, resulting in taking twice, or more, of the time in making a power adjustment.
In order to continuously record moving pictures onto the optical disk, it is necessary to record them at a predetermined transfer rate. Accordingly, during a time period when a power adjustment is made while the recording operation is temporally stopped the data for the moving pictures must be stored in the DRAM. Taking twice, or more, of the time in making a power adjustment leads to taking twice, or more, of the capacity of the DRAM, which results in increasing the cost of the optical disk apparatus.